The regulation of glycogen metabolism has been studied in C-6 astrocytoma and C-1300 neuroblastoma cell lines in culture. There appear to be two principal modes of controlling glycogen synthesis and breakdown. The concentrations of glycogen in the cells, and the amount of glycogen phosphorylase and glycogen synthase in the active forms can be regulated by the available glucose supply; however, there is a maximum level of glycogen which is not exceeded even in the presence of greatly increased glucose supply. When glucose in the medium falls below a critical level, phosphorylase is activated and glycogenolysis occurs. Substances which cause cyclic 3', 5'-AMP to be elevated in the cells also stimulate glycogenolysis. In C-6 glioma cells, norepinephrine is effective; in C-1300 neuroblastoma cells, prostaglandin E, is effective. Both cell lines respond to the phosphodiesterase inhibitor, isobutyl methylxanthine, and the adenyl cyclase stimulator adenosine.